As known, a power factor correction (PFC) circuit is widely used in a power supply system of an electronic apparatus for filtering off the harmonic wave component contained in the received current, thereby increasing the power factor. Consequently, the power supply system comprises a power module with a power factor correction circuit. By the power factor correction circuit, the distribution of the AC input current provided by the utility power source is no longer too centralized, the magnitude of the harmonic wave is reduced, and the power loss of the power supply system is reduced.
As the demands on the power level and the backup function of the electronic apparatus are gradually increased, the power supply system is usually equipped with two or more power modules. Conventionally, each of the power modules comprises a power factor correction circuit and a DC-to-DC converter. The output sides of the DC-to-DC converters of these power modules are connected with each other in parallel and further electrically connected with a main circuit of the electronic apparatus. Moreover, for providing sufficient electric energy to the electronic apparatus, each power module may be removed from or plugged into the power supply system during the electronic apparatus is in the normal working state. In other words, the plural power modules of the power supply system are hot-swappable to be plugged into the power supply system.
However, when the plural power modules are plugged into the power supply system in a hot-swappable manner to be connected with each other in parallel, two problems may occur. For example, when plural power sources provide electric energy to the plural power modules, if the phases of different power sources are different or there is a voltage difference between the plural power modules, the output current from the working power module will flow into a specified power module which is being plugged into the power supply system in the hot-swappable manner. Under this circumstance, an unexpected returning current path is generated in the swapping power module. Due to the unexpected returning current path, the filtering efficacy of an EMI filter of the swapping power module is deteriorated, and the pre-charged voltage across an output capacitor of the swapping power module is too high.
Moreover, when a specified power module is plugged into the power supply system in the hot-swappable manner to be connected with the working power module in parallel, since the voltage across the output capacitor of the swapping power module is zero at the moment of plugging the swapping power module into the power supply system, the output voltage from the working power module of the power supply system will charge the output capacitor of the swapping power module. Under this circumstance, the output capacitor of the swapping power module is readily subjected to a large dv/dt change, and thus an inrush current is generated by the output capacitor. Due to the inrush current, the power module may be damaged.
For solving the above drawbacks, the power supply system may be equipped with an isolation transformer. The isolation transformer comprises plural primary windings and a secondary winding. The plural primary windings are electrically connected with the output sides of the corresponding power modules. The secondary winding is electrically connected with the main circuit of the electronic apparatus. By the isolation transformer, the plural power modules are isolated from each other. Consequently, when any power module is plugged into the power supply system in the hot-swappable manner, the possibility of generating the unexpected returning current path and the inrush current will be reduced. However, since the isolation transformer is bulky, the overall volume of the power supply system fails to be effectively reduced. Moreover, since the power supply system has the additional isolation transformer to transmit electric energy, the use of the isolation transformer may result in undesired power loss. Under this circumstance, the efficiency of the power supply system is impaired.
Therefore, there is a need of providing a power supply system and a control method thereof in order to eliminate the above drawbacks.